The present invention relates to a brake unit for automotive vehicles comprising a fluid-actuated brake booster and a master brake cylinder secured thereto, at whose end remote from the brake booster the brake-pedal force is introduced. The brake-pedal force is transmitted via a pressure member penetrating a piston of the master brake cylinder onto a control element of a control valve disposed in the brake booster, which controls the fluid supply to the brake booster. A tie element transmits the brake-boosting force from a fluid-actuated movable wall of the brake booster onto the piston of the brake master cylinder, and with a reaction device which--dependent upon the brake-boosting force--transmits a reaction force directed in opposition to the brake-pedal force onto the pressure member.
Brake units of this type serve to boost the pedal force applied by a brake pedal and to transform it to a hydraulic actuating pressure generated in the brake master cylinder for one or several brake circuits. In contrast to a travel-responsive brake-boosting operation, in which only the actuating position of the brake pedal determines the magnitude of the braking force generated, without the driver getting a feeling for the magnitude of the braking force generated, in a force-responsive brake booster, a reaction force responsive to the generated braking force will be produced through the reaction device, which counteracts the pedal force and gives the driver a feeling for the magnitude of the braking force generated. At the same time, this reaction force operates, in opposition to the actuating direction, on the control valve initiating the brake-boosting operation, so that a boosting effect initially achieved will be interrupted, if the driver does not move the brake pedal further in opposition to the reaction force occurring by applying a higher amount of pedal force.
In a brake unit of the type referred to hereinabove, such as, for example, U.S. Pat. No. 2,929,216, the actuating member or pressure member is constructed as a push rod which is encompassed by a tie sleeve forming the tie element. The reaction device includes an annular piston subjected to the hydraulic brake pressure generated, which piston transmits the reaction force via a compression spring and, after having overcome the spring force, via a stop onto the push rod and thus--in opposition to the pedal force--onto the brake pedal. The known brake unit which incorporates a high-pressure brake booster is of a comparatively complicated structure. The reaction force obtained by hydraulic transmission acts in two steps and, therefore, is not proportional to the braking force generated. Since a separate piston, which is located in the brake master cylinder, is necessary for the generation of the reaction force, the space required in the brake master cylinder and the structural space needed therefore will be essentially increased.
Since in the case of the known brake unit, the reaction force is generated by direct application of hydraulic pressure on an effective surface connected with the push rod, this brake unit is only able to be employed for a single-circuit brake master cylinder. In a dual-circuit brake master cylinder, each of the two brake circuits would have to be provided with a separate hydraulic effective surface at which the reaction force is generated. If the hydraulic effective surface destined to produce the reaction force were located in one of the two brake circuits only, no reaction force at all would be generated upon failure of this brake circuit. The arrangement of two effective surfaces each assigned to a different one of the two brake circuits, would, at the most, be possible to be obtained by great structural effort. However, there would in any case take place a substantial decrease in the reaction effect upon failure of one of the two brake circuits.
In brake units known in various designs, for example German Patent DE-OS No. 2,837,911, the brake master cylinder is located on the side of the brake booster remote from the pedal side. In this arrangement, a reaction disc composed of a rubber-elastic material is employed to generate the reaction force reacting on the brake pedal, which reaction disc is compressed at its outer periphery by the force transmitted between the movable wall of the brake booster and a push rod leading to the brake master cylinder. The material displaced by the quasi-hydraulic behavior of the reaction disc presses against a control valve piston force-transmittingly connected with the brake pedal and generates the reaction force sensed by the driver.
This known reaction device is of a comparatively straightforward design. It hardly changes the overall length of the brake unit. However, difficulties are encountered with the length adjustment required for the compensation of tolerances. With this type of a brake unit, problems arise by the brake master cylinder being arranged on the side of the brake booster remote from the pedal side, so that considerable forces have to be transmitted via the housing of the brake booster, or via separate tension-transmission elements which may be constructed, for instance, as tie rods or as a central tube.